The present invention relates to an ultrasonic measuring apparatus, and more particularly to an ultrasonic measuring apparatus for applying crossing ultrasonic beams to a subject, thereby measuring the sound velocity of the ultrasonic waves propagating through the subject.
Various methods have been proposed wherein ultrasonic waves are applied to the region of interest of a living subject to obtain data showing the conditions of tissues in the region of interest. Of these methods, the so-called cross beam method is drawing particular attention. In this method, an ultrasonic transmission beam and an ultrasonic echo beam are applied, crossing each other, to a subject to obtain various acoustic parameters and various nonlinear parameters showing the condition of the region of interest.
Ultrasonic diagnosis apparatuses employing the cross beam method are disclosed in U.S. patent application Ser. No. 835,195 and also in the U.S. patent application filed on June 18, 1986, claiming priority based on Japanese Patent Application No. 60-133421. Both apparatuses use a linear steering type ultrasonic transducer. The ultrasonic transducer of this type comprises an array of a number of ultrasonic transducer elements. The elements are divided into a first group and a second group. These transducer groups are set apart for a predetermined distance. When the first transducer group is used as a beam transmission unit, drive pulses generated at different times are supplied to the ultrasonic transducer elements of the first group. Driven by these pulses, the transducer elements emit an ultrasonic beam at a specific angle to a region of interest. The second transducer group receives the ultrasonic waves reflected from the region of interest, called "echoes." The period between the emission of the ultrasonic beam and the receipt of the echo wave, which corresponds to the beam, is measured. From this period, or the propagation time, the sound velocity of the ultrasonic wave propagating in the subject is calculated. This sound velocity is used in diagnosing the subject.
In the conventional ultrasonic diagnosis apparatus described above, the signal obtained by each beam transmission and each wave reception contains unwanted components called "ripples." The ripples are generated when the ultrasonic beam scatters, causing a phenomenon known as "speckle," at points in the region of interest, other than the point which the axis of the beam passes. Due to the ripples, the propagation velocity of the ultrasonic wave cannot be accurately measured. In order to eliminate the ripples, the transducer array can be moved while transmitting beams and receiving echoes, thus obtaining more data, and the data can then be averaged. For the same purpose, a large quantity of data can be obtained from the echoes reflected from the tissues moving due to the heart beats or the motion of the lungs, and is then averaged.
The first method of eliminating the ripples is disadvantageous in two respects. First, the distance between the subject and the ultrasonic transducer array inevitably changes, whereby the data obtained is not sufficiently accurate. Secondly, the operator tends to move the array away from a B-Mode image observation region.
The second method of eliminating speckles is also disadvantageous. Although the second method does not have the problems inherent in the first method, it cannot provide sufficient data within a short time since the tissues cannot move as quickly as desired.